Experimental Study on the Slit Glazed Solar Air Heater

The energy demand all over the world has been continuously increased. The ever increasing cost, limited resources and possible environmental risks of using conventional energy resources have increased the renewable energy utilization. Solar energy is considered the most promising, inexhaustible and plentiful renewable source of energy. Solar collectors are employed in converting solar energy into thermal energy. The energy consumption of space heating in residential and industrial sector is considerable. In solar energy utilization direct use for space heating could be achieved by employing solar air heaters (SAHs). Technical feasibility has been achieved for SAHs. One of the main concerns is providing economic feasibility. The present study consists of three series of experiments. First, three SGSAHs with different bed heights (7 cm, 5 cm, and 3 cm) were fabricated with multiple glass panes used for glazing. The length, width, and thickness of each pane were 131 cm, 6 cm and 0.4 cm respectively. The mass flow rate was varied between (0.014 and 0.057) kg/s. The second series of experiments had been conducted to investigate the effect of the width of glass panes, the distance between the slits and mass flow rates on the efficiencies of the three SGSAHs. A glass pane with the length of 131 cm and thickness 0.4 cm was studied for three different glass pane widths: 6 cm, 5 cm, and 4 cm. In addition, the bed height of duct in the second series of experiments was fixed at 7 cm. The ambient air was continuously withdrawn through the gaps between the glass panes by fans. In the first and second series, the experiments were conducted for four different gap distances between the glass panes (i.e., 0.5 mm, 1 mm, 2 mm and 3 mm). Finally, the third series of experiments had been carried out on a SGSAH and a UTC to compare their efficiencies. The mass flow rates in the second and third series of experiments were varied between (0.014 and 0.029) kg/s. In the first series of experiments the highest efficiency obtained was 82% where the bed height was 7 cm, glass pane gap distance was 0.5 mm, and the mass flow rate was 0.057 kg/s. The air temperature difference between the inlet and outlet (∆T) was maximum (27˚C) at the lowest mass flow rate (i.e., 0.014 kg/s) for the same SGSAH. The results demonstrated that for mass flow rates lower than 0.036 kg/s and gap distances greater than 2 mm, the performance of the SGSAH with 3 cm bed height was better compared to the SGSAHs having 5 cm and 7 cm bed heights. However, for flow rates equal or higher than 0.036 kg/s, the SGSAH with 7 cm bed height performed better for all different gap distances compared to the other two SGSAHs. The experimental results of the second series indicated that the maximum thermal efficiency of 75% was obtained when the gap distance was 0.5 mm, slit width was 4 cm and the mass flow rate was 0.029 kg/s. The maximum rise in air temperature was noted as 28℃ at the lowest mass flow rate where the gap distance and slit widths were 0.5 mm and 4 cm respectively. The experimental results obtained from the last series indicated that the thermal efficiency of the SGSAH was 16% higher than the UTC’s efficiency. Keywords: Slit glazed solar air heater, thermal efficiency, unglazed transpired collector, bed height